[Support] Fix for two issues with clearing of the internal storage for cl::bits

This patch fixes two issues with clearing of the internal storage for cl::bits

1. The internal bits storage for cl::bits is uninitialized. This is a problem if a cl::bits option is not defined with static lifetime.
2. ResetAllOptionOccurrences does not reset cl::bits options.

The latter is also discussed in:

https://lists.llvm.org/pipermail/llvm-dev/2021-February/148299.html

Differential Revision: https://reviews.llvm.org/D119066